An insight in to microwave induced defects and its impact on nonlinear process in NiO nanostructures under femtosecond and continuous wave laser excitation

This work demonstrates the impact of microwave (MW) irradiation on third-order nonlinear optical (NLO) processes in chemically deposited NiO nanostructure films. The optical nonlinearity of the NiO nanostructure films was studied using third-harmonic generation (THG) measurements in the pulsed femtosecond laser regime and the Z-scan technique in the continuous wave laser regime. In the ultrafast pulsed regime, THG measurements revealed a significant increase in the THG signal of MW-irradiated NiO nanostructures due to photoexcitation and relaxation processes, resulting from an enhancement in defect concentration. This increase in defect density upon MW irradiation was quantified by PL and XPS studies. Under continuous wave laser irradiation, the Z-scan technique showed an enhanced absorption coefficient of ∼10−1 m W−1 and a nonlinear refractive index of ∼10−7 m2 W−1. The high NLO values in both pulsed and continuous laser regimes indicate that MW-irradiated NiO nanostructure films hold promise for optoelectronic device applications.


Introduction
Nanostructure lms are pivotal in the realm of photonics due to their extensive range of optical applications.The nanoscale dimensions of these lms signicantly enhance their optoelectronic properties due to unique physical and optical characteristics at this scale.Nanomaterials exhibit size-dependent properties, including increased surface-to-volume ratios, quantum connement effects, and enhanced light-matter interactions, thereby improving electron-photon interactions. 1,2hese characteristics lead to improved performance in various promising elds, such as photonics, optoelectronics, telecommunications, biomedical optics, and nonlinear optics (NLO).
4][5] These studies have revealed the exceptional capabilities of nanomaterials for multiphoton absorption and high nonlinear absorption coefficients.These properties contribute to strong and swi responses in NLO materials, making them ideal for advanced applications.0][11] NiO semiconductor stands out among various transparent metal oxides due to its wide bandgap and electronic, optical, and magnetic properties, making it valuable for interdisciplinary applications.Known for its high stability, both chemically and physically, as well as its low toxicity, and durability, NiO has a signicant impact in various elds.In optoelectronics, NiO is recognized for its role in solar cells, 12 photodetectors, 13 and light-emitting diodes. 14When subjected to intense, coherent laser beams, NiO nanostructure lms exhibit remarkable nonlinear optical properties. 9,15ost-deposition treatment techniques such as annealing and irradiation have been extensively studied for their ability to enhance the properties of NiO materials.High-energy beams with high-frequency irradiation, such as electron beams, gamma rays, and ultraviolet rays, are powerful tools for modifying the microstructural properties of materials.These beams have been extensively explored in various applications, particularly in the elds of electronics, sensing, and optoelectronics. 16,17They offer several advantages, such as the capacity to conveniently adjust the optical and structural properties of materials.Electron beams, gamma rays, and ultraviolet rays can induce signicant changes in the properties of materials, making them highly desirable for a diverse array of applications.In addition to their utility in modifying the properties of materials, high-energy beams also offer several practical advantages.For instance, they can be precisely controlled to achieve the desired level of modication, allowing for netuning of the material's properties.Numerous studies have reported that irradiation modies the physical, chemical, and optical properties of NiO nanostructure lms, leading to enhanced optical responses.In view of the mentioned distinctiveness of NiO lms, many researchers have been attempting to explore their structural, optical, and NLO properties.M. Rashad et al. 17 synthesized NiO nanoparticles via microwave irradiation and observed that UV exposure signicantly reduces the optical bandgap by creating defects and disorders.They also noted that the nanoparticles exhibited enhanced optical nonlinearities associated with quantum size effects.P. Mallick et al. 18 prepared NiO thin lms on Si substrates using the electron vaporization method and subjected them to swi ion irradiation.The authors observed that swi ion irradiation improved the crystallization and texturing of the lms at intermediate uences, with changes in the crystallinity and texturing of NiO grains depending on the initial microstructure of the lm. A. Qasem et al. 19 investigated nanostructured NiO thin lms treated with laser pulses ranging from 0 to 150, with intervals of 50 pulses.The energy band gaps were found to be lowered and the band tail energies to be extended upon exposure of laser light.SEM micrographs showed holes and cracks due to the laser breaking bonds between Ni 2+ and O 2− ions.The calculations conrmed the optical bandgap contraction and tail energy expansion with increasing pulses, attributed to increased defects.K. Jouini et al. 20 prepared NiO thin lms using spray pyrolysis and subjected them to gamma irradiation ranging from 180 Gy to 10 kGy.Structural studies revealed a phase transition from NiO to Ni 2 O 3 beyond 5 kGy.At a 10 kGy dose, the lms exhibited a combination of NiO and Ni 2 O 3 phases, with optical studies showing activity in both the UV and visible ranges.This combination enhanced the photocatalytic performance under gamma ray treatment.Recent research has demonstrated that various irradiation methods can signicantly modify the structural, optical, morphological, and nonlinear optical properties of NiO thin lms.The ndings from these studies conrm that irradiation enhances the material's properties.
Furthermore, these beams can be utilized across a diverse array of materials, establishing them as versatile tools in materials science and engineering.But sometimes, these high energy beams are found to destroy the materials also.This is where microwave (MW) irradiation, a low-frequency irradiation, offers a less destructive and more cost-effective approach to material modication.MW irradiation has emerged as a powerful tool in various elds due to its unique advantages over conventional heating methods. 21It offers several advantages over other forms of irradiation, such as rapid and uniform heating, higher yields, and reduced energy consumption. 22hese benets have led to its widespread adoption in research and industrial applications.Here, in this study, we are going to explore the new novelty eld of the inuence of MW irradiation on NiO nanostructure lms.In this study, we need to emphasize the method that we employed to prepare for nanostructure NiO lms.We utilized the air-assisted chemical spray pyrolysis method to prepare NiO nanostructured lms.This method is renowned for its ability to produce uniform lms and is favored for its simplicity and cost-effectiveness.Several deposition parameters include the ow rate of the spray, the amount of solution sprayed onto the substrate, the substrate temperature, and the distance between the nozzle and the substrate signicantly inuence the quality and uniformity of the nanostructure lms.Optimizing these parameters is crucial for producing lms, which attributes distinguish it from other deposition methods, making it a highly attractive option for the deposition of NiO nanostructure lms.
The literature on third-order nonlinear optical effects in NiO nanostructures is limited.Additionally, comparative studies on the inuence of NiO nanostructure lms under continuous wave and pulsed laser regimes are not well-established.To address this gap, our recent study focused on synthesizing NiO nanostructure lms using spray pyrolysis, followed by an extensive examination of their NLO response in NiO nanostructures using both CW and femtosecond pulsed lasers.Importantly, we aim to understand the NLO processes in systems treated with microwave irradiation and examine the impact of MW irradiation on both CW and pulsed laser regimes.Therefore, this research investigates the impact of microwave irradiation on the nonlinearity and thermo-optic properties of NiO nanostructure lms, utilizing both pulsed and continuous wave laser regimes.

Materials used
Microscopic glass slides were acquired from Labtech.Acetone, 2-propanol (EMPLURA grade), and nickel acetate tetrahydrate (98% purity) were sourced from MERCK.Deionized water (18.3MU) was used throughout all procedures.All chemical compounds were utilized to prepare the precursor solution for spray pyrolysis technique.

Nanostructure thin lm deposition
Microscopic glass slide substrates were cleaned sequentially with soapy water, deionized water, acetone, and isopropanol (IPA) for 10 minutes each using an ultrasonicate.Subsequently, the substrates were dried under a nitrogen ow.To ensure the removal of organic contaminants and reduce surface roughness, the substrates were then subjected to ozone treatment for 10 minutes, resulting in a cleaner and more uniform surface.
The aqueous precursor solution was prepared by dissolving 1.2442 g of nickel acetate tetrahydrate (Ni(CH 3 COO) 2 $4H 2 O) in 100 mL of deionized water, followed by stirring for one hour to ensure a homogeneous and clear solution.This solution was then loaded into the spray pyrolysis syringe for direct use in NiO deposition.The substrate temperature was maintained at a constant 450 °C during the deposition process.The spray pyrolysis soware controlled the spray head's motion along both the x and y axes.The solution was sprayed onto the glass substrate placed on the substrate holder, where it underwent pyrolysis, forming a thin lm.The spraying nozzle and the substrate were spaced 19 cm apart with a solution being sprayed ow rate of 2 mL min −1 .Air served as the carrier gas at 1 mbar.
With 2 mm intervals, the ultrasonic nozzle moved in an Sshaped pattern in the x and y axes, maintaining a steady speed of 30 mm s −1 .During deposition, the temperature varied by ±10 °C around 450 °C.
The nickel acetate tetrahydrate precursor solution decomposed and pyrolyzed at 450 °C, resulting in solid nickel oxide, while carbon dioxide and water were released as gases.
The resulting lm is light brown in color.The thickness of the lm was ascertained through the utilization of a stylus prolometer, the lm thickness was determined to be ∼250 nm.

Microwave irradiation
The NiO nanostructure lms were subjected to microwave irradiation using a commercially available microwave oven (LG, model number: MC2146BRT) operating at a frequency of 2.45 GHz.The samples were positioned at the center of the microwave chamber on a rotating table.The microwave power was xed at 800 W for irradiation.The irradiation time varied between 2, 3, 4, 5, and 10 minutes respectively.Aer each irradiation, the sample was allowed to cool, and it was observed that the sample became darker grey as the irradiation increased.Fig. 1 illustrates the changes observed in NiO nanostructure lms upon MW irradiation.The schematic representation of process ow for preparation of microwave irradiated NiO nanostructure lms, depicted in Fig. 2.

Characterization of nanostructure lms
2.4.1.Structural characterization.The deposited lms' crystal structure was studied using a Rigaku SmartLab diffractometer with Cu Ka radiation with 40 kV and 30 mA, scanning from 30°to 90°at a rate of 1°min −1 .The deposited lms underwent analysis for their composition, phase, impurity levels, and defects using LabRAM HR (UV) Raman spectroscopy with a 514 nm laser and CCD detector at room temperature.
X-ray photoelectron spectroscopy (XPS) was utilized for the assessment of the oxidation state of the elements in the deposited lms.The analysis was performed using an AXIS ULTRA system with a monochromatic Al Ka X-ray excitation source operating at 14 kV.The C 1s signal at 284.8 eV was used as the reference binding energy.
2.4.2.Morphological and optical characterization.Both Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM) were employed to study the morphology and surface features of the deposited thin lms.FESEM analysis was conducted using a ZEISS ULTRA55 scanning electron microscope with a dispersive energy X-ray (EDX) analyzer, operated at 20 kilovolts, aer sputtering a thin layer of gold onto the samples.AFM observations were made using an Innova SPM atomic force microscope in tapping mode.
The nanostructure deposited lms were analyzed for UVvisible light transmission using a 1900i UV-vis spectrophotometer, ranging from 190 to 1100 nm, with glass substrates providing the background.Photoluminescence studies were conducted to detect defects in the lms, using a JASCO FP-8300 spectrouorometer.Measurements were taken from 320 nm to 750 nm at ambient temperature, with an excitation wavelength of 300 nm, acquiring an emission spectrum for the thin lms.
2.4.3.Third-order nonlinear optical processes.To explore the nonlinear optical properties of the deposited lm, we conducted NLO experiments using Z-scan techniques.When a high-intensity laser passes through a material, it exhibits nonlinear behavior concerning the electric eld.To study the effects of lms when subjected to laser light, we conducted a Zscan experiment. 23Fig. 3 depicts the conguration of the Z-scan experiment.The experimental setup of the Z-scan technique includes a laser, convex lens, translating moving sample holder, aperture, and a detector.The experiment involves two methods to determine the nonlinear absorption (NLA) and nonlinear refraction (NLR) of the material.The open-aperture Z-scan method provides information on NLA, while the closedaperture method gives the NLR of the material.In this experiment, we used a continuous wave He-Ne laser known for its high stability and low noise.Operating at a xed single wavelength of 632.8 nm, it offers a consistent and well-characterized light source for nonlinear optical studies.The laser light passes through the convex lens and then the sample-translating stage.A convex lens with a focal length of 5 cm is used to focus the laser beam onto the sample.The sample was moved from −5 cm to +5 cm in both z-directions around the focal point of the convex lens using a linear translation stage.The laser light that passes through the sample is detected by the detectors.These detectors are crucial for accurately measuring the transmitted laser power.They are connected to a power meter (THOR LABS PM320E), which in turn is connected to a PC for data recording and analysis.The movement of the linear translator is controlled by the Z-scan soware.In the open-aperture (OA) conguration, the laser beam is directed through a convex lens focus it onto the sample.The detector measures the entire transmission through the sample.In the closed-aperture (CA) conguration, the beam that has passed through the sample is directed through an aperture positioned ahead of the detector.This aperture is placed to ensure that any nonlinear phase shi experienced by the beam is solely due to the sample.In the current study, the normalized transmittance value S = 0.7.
In this report, the NiO nanostructure lm sample is irradiated by an intense continuous wave He-Ne laser with an input intensity of 20 mW.During the experiment, the beam is passed through a 5 cm convex lens, which focuses the beam on the sample.The obtained beam waist at the focal point and Rayleigh length were 35.11 mm and 6.11 mm, respectively.The thin lm thickness meets the sample requirement L ( Z R , in which the Rayleigh range (Z R ).The sample thickness is less than the Rayleigh length Z R, hence the thin lm approximation is valid.Then, Z-scan experiments were conducted on the plane glass substrate, revealing no signicant nonlinear contribution at the applied input power levels.Consequently, the glass substrate's impact on the observed nonlinearity was deemed negligible.A linear translation stage was employed to scan the NiO thin lm in both positive and negative z-directions surrounding the convex lens's focal point.The scan range was ±5 cm.The laser power transmitted through the NiO lm is utilized to measure both the nonlinear refractive index n 2 and the nonlinear absorption coefficient b, with and without aperture, in front of the detector, accordingly.The detector is connected to power meters, utilized for measuring the laser power that passes through.
2.4.4.Third harmonic generation.The third harmonic signal of the light beam from deposited nanostructured NiO thin lms was measured using the third harmonic generation (THG) technique.Third harmonic signals are generated when a fundamental frequency beam passes through a nonlinear material, producing a frequency that is three times that of the input.To  investigate the third harmonic response of the material, we conducted measurements using the THG method, as depicted in Fig. 4. The experimental setup comprises two pulsed lasers: a femtosecond Yb:YVO 4 laser with a wavelength of 1045 nm and a nanosecond Nd:YAG laser with 1064 nm wavelength.The setup also includes a chopper, Glan's polarizer, mirrors, a silicon photodetector, a rotating sample holder, an interference lter for 355 nm wavelength, a photomultiplier tube, an oscilloscope, and a PC for data acquisition and analysis.
In this experiment, to determine the third harmonic signal of the material, we employed a femtosecond (fs) laser as the pumping laser.The fs laser used was a Yb:YVO 4 laser with a wavelength of 1045 nm, a repetition frequency of 63 MHz, and a maximum energy of 40 nJ.This fs laser induces changes in the dipole moment of the sample, thereby altering the electronic states of the material.The high intensity and ultrashort pulses of the fs laser generate high peak powers necessary for strong nonlinear interactions.To prevent thermal damage to the samples due to prolonged exposure to the fs laser, a chopper was placed in the path of the laser beam.The chopper, with a single slit 2 mm wide, rotates at a speed of 1 revolution per second, effectively modulating the pulsed laser beam that falls on the sample.A nanosecond (ns) laser was also utilized, with its spot size corrected to approximately 1 mm to match that of the femtosecond laser, ensuring that both beams interact with the same area of the sample.This uniformity is crucial for consistent and reliable third harmonic generation (THG) measurements.The ns laser served as the probing laser beam, detecting changes induced by the pumping beam.The probing beam used was an Nd:YAG laser with a wavelength of 1064 nm, an 8 ns pulse duration, and a repetition frequency of 10 Hz.The laser beam had a diameter of 8 mm and a maximum energy of 150 J m −2 .
The laser beam then passed through a Glan's polarizer placed in front of the beam path.The Glan polarizer ensured that the probing beam (from the ns laser) was highly polarized and allowed modulation of the light beam's intensity reaching the sample, leading to the tuning of the output fundamental energy density.The polarizer was capable of withstanding laser damage at 4 GW cm −2 .We adjusted the polarizer angle in 5 degree increments to control the 1064 nm laser's energy reaching the NiO nanostructure lm.This adjustment allowed us to detect the THG signal at the lowest possible energy density of the ns laser.
Following, the light passed through a transmissive mirror that reected a portion of the incident light while transmitting the rest.This mirror directed part of the ns laser beam to a silicon photodetector (Si PD) and allowed another part to pass through to the sample surface.The reective portion of the mirror directed a segment of the probing beam to the Si PD, used to synchronize the measurement system.The transmissive portion of the mirror allowed the probing beam to reach the sample surface, where it interacted with the changes in optical dipole moments induced by the fs pumping laser.The resulting THG signal then passed through an interference lter with a wavelength of 355 nm, which exclusively allowed the THG signal to pass through.
The THG signal was then recorded by a photomultiplier tube followed by an oscilloscope.The Tektronix MSO 3054 oscilloscope, with a high sampling rate of 2.5 GS s −1 , provided highresolution measurements of the fast-changing signals from the laser interactions, capturing both the fundamental signal (1064 nm) and the THG signal.The oscilloscope was connected to a PC for data acquisition and analysis.The whole experimental system was bounded in a box to eliminate the impact of external unwanted light dispersion, ensuring that the THG measurements reected the true interactions within the sample, providing reliable and high-quality data.

XRD analysis of MW irradiated NiO nanostructure lms
To investigate the crystal structure and phase of the prepared nanostructured lms, structural studies were conducted using X-ray diffraction (XRD) analysis.The XRD patterns were  222) planes.The polycrystallinity is evident in all samples.The preferential peak is towards the (200) plane, which is the most intense peak.Increasing the MW irradiation led to noticeable alterations in both peak intensity and full width at half-maximum (FWHM).Specically, the intensity decreased with 5 minutes of MW irradiation but increased again for the 10 minute irradiated sample.These variations are attributed to the MW heating effect, which inuences the orientation and arrangement of the lattice planes.Furthermore, the smaller plane peaks diminished with increased MW irradiation.The broadening of FWHM suggests an increase in strain within the lm due to the MW heating effect.The observations, such as changes in intensity and FWHM, validate the structural alterations occurring in NiO thin lms during MW exposure.These variations in intensity and FWHM may be attributed to the increased defect concentration resulting from dislocation or distortion within the lm induced by irradiation. 24 minimal peak shi (∼0.13°) towards lower angles was observed in samples irradiated for 2 to 10 minutes compared to the pristine samples, implying thermal expansion in the lattice parameters caused by MW heating.Furthermore, this alteration in the diffraction peak position is associated with changes in the local atomic environment of the Ni 2+ ions within the crystal lattice.Such changes can be attributed to structural modications, such as lattice distortion, strain, 25 or the introduction of defects induced by MW irradiation.No additional peaks, which could indicate secondary phases or impurities, were detected aer MW irradiation, conrming the purity of the samples.These ndings demonstrate that MW irradiation affects the lattice atoms orientations and strain within the NiO thin lms without introducing impurity phases.
The structural properties of the prepared nanostructured lms, specically the crystallite size and microstrain, were determined using two prominent analytical methods: the Scherrer method and the size-strain plot (SSP) method.
3.1.1.Scherrer method.The Scherrer method estimates the crystallite size within the material based on the peak broadening observed in the X-ray diffraction (XRD) pattern, primarily attributing this broadening to the nite size of the crystallites.The crystallite size (D) is calculated using the Scherrer equation: where l is the wavelength of the XRD radiation, b is the full width at half maximum (FWHM) of the peak, q is the Bragg angle, k is the shape factor, typically close to 0.9 for spherical particles.
3.1.2.Size-strain plot.The SSP method enables the simultaneous calculation of crystallite size and micro strain, assuming that peak broadening results from both crystallite size and strain effects.In this method, the size-induced broadening is described by a Lorentzian prole, while the strain-induced broadening follows a Gaussian prole.The SSP method utilizes the following equation: where d hkl is the interplanar spacing, b hkl is the FWHM of the peak corresponding to the (hkl) planes, 3 represents the microstrain.
In the size-strain plot, (d hkl b hkl cos q) 2 is plotted on the Y-axis and (d hkl 2 b hkl cos q) on the X-axis, is depicted in Fig. 6.The slope of the plot provides the crystallite size, while the intercept gives the strain within the lms.The SSP method focuses more on reections at lower angles, where the accuracy and precision of XRD data are typically higher.
The crystallite sizes calculated using both the Scherrer method and the size-strain plot method are nearly equal, and both methods show consistent trends.The SSP method is more reliable and appropriate for analyzing the NiO nanostructure lms because it minimizes the inuence of less precise data and shows a good linear t, indicating isotropic variations accurate in the crystallite size and strain of the NiO nanostructures upon MW irradiation.The crystallite size decreases with increasing microwave irradiation, from 8.24 nm to 7.47 nm according to the Scherrer method, and from 8.48 nm to 7.76 nm according to the SSP method.This reduction in crystallite size is attributed to the breaking up of crystallite size due to the MW heating effect.Irradiation can lead to the generation of heat in the material and inuence the mobility of atoms in the crystal lattice, affecting the growth and stability of crystallites. 26,27Consequently, the irradiation period induces alterations in the lm's microstructure by disrupting its original orderly arrangement. 17he dislocation density, which is a measure of point defects in the material, represents the number of dislocations per unit length and is calculated using the formula: Aer MW irradiation, the dislocation density of the lm was found to increase, indicating that the levels of dislocation, distortion, and defects within the lms have been enhanced.This increase is attributed to the MW heating effect, which induces more dislocations in the lattice.
The lattice parameter for the cubic structure is given by: where (hkl) are the Miller indices of the planes corresponding to each peak in the XRD patterns.Observations show that there is no signicant change in lattice parameters with increased irradiation.Any small changes in the lattice parameter may be due to lattice compression induced by thermal expansion from the MW heating effect.Strain within the lms was calculated using the equation: The lms exhibit an increase in strain, which is associated with the elevated presence of defects induced by MW irradiation.However, a considerable difference in strain values was observed between the Scherrer and SSP methods.This indicates that the SSP method offers a more comprehensive analysis by considering strain.The number of crystallites per unit volume was determined using: where t is the lm thickness.An increase in the number of crystallites aer MW irradiation suggests that MW irradiation effectively modies the microstructure of the nanostructure lms.
The structural parameters values are depicted in Table 1 and it reveals that MW irradiation has been shown to increase dislocation density, induce strain, and enhance the number of crystallites in the lms.These changes reect signicant modications to the lms' microstructure, which can inuence their overall physical and chemical behavior.

Raman analysis of MW irradiated NiO nanostructure lms
Raman spectroscopy was employed to investigate the vibrational modes and structural properties and to identify   3.2.1.First-order Raman phonon modes.The rst-order Raman phonon modes, including the Transverse Optical (TO) mode at 380.6 cm −1 and the Longitudinal Optical (LO) mode at 492.8 cm −1 , are observed due to parity-breaking bonds caused by nickel vacancies at the zone boundary.Typically, stoichiometric NiO is inactive for rst-order phonon modes.The existence of these modes in the spectra indicates non-stoichiometry in the material, attributed to nickel defects including nickel vacancies, 28 or the presence of Ni 3+ ions in the lms. 29Additionally, they are induced by oxygen interstitials, surface defects, and imperfections occurring in the nanocrystalline lms. 30The LO bands arise from Ni-O bonding, specically related to the stretching bonds in the NiO octahedra.Microwave irradiation enhances the intensity and area of the Raman LO mode, indicating its interaction with NiO materials.This irradiation induces a stochastic distribution within the NiO lattice, increasing disorder and enhancing phonon scattering, which broadens the Raman peaks.
The TO phonon modes exhibit a redshi from 380.6 to 423.5 cm −1 upon MW irradiation, with the FWHM increasing from 47 to 216 cm −1 .The area of the TO mode is highest for the 2 minute irradiated sample, suggesting signicant lattice distortion or the presence of nickel defect levels, corroborated by XRD spectra.The LO phonon modes shi from 492.8 to 577.8 cm −1 , with an increased FWHM, attributed to the interaction of MW irradiation with NiO material.Similar observations have been reported by A. Sunny et al. 31 The shis observed in the TO and LO modes indicate the presence of vacancies and structural defects induced by MW irradiation. 32The movement of Raman peaks towards higher wavenumbers suggests an increase in defect concentration within the lm aer MW irradiation, possibly due to lattice defects introduced by the MW heating effect on the NiO structure. 33.2.2.Second-order Raman phonon modes.The secondorder Raman vibration modes, 2TO, TO + LO, and 2LO, result from combinations of rst-order bands. 34For the pristine sample, the TO + LO and 2LO bands occur at 928.8 and 1068.0 cm −1 , respectively.The 2LO mode also indicates stretching vibrations of the Ni-O bond in the NiO lattice. 35For 10 min irradiated sample, new peaks characteristic of NiO arise at 796.4 cm −1 , attributed to the 2TO mode.The weak intensity of the 2TO and TO + LO modes suggests that the synthesized materials have a nanocrystalline structure. 36Additionally, the intensities and areas of the 2LO mode non-monotonically increase upon MW irradiation, conrming enhanced disorder or defect presence in the lms and aligning with earlier XRD results.The dependence of MW irradiation on phonon mode frequencies and FWHMs is summarized in Table 2.
In the pristine sample to the 5 minute MW-irradiated sample, the one-phonon 1LO mode exhibits higher intensity due to the presence of defects or surface defects.However, as the irradiation is extended to the 10 minute sample, the two-phonon LO band becomes more broadened compared to the 1P mode band.This increased broadening indicates higher disorder and defect levels within the nanostructure lm.Additionally, the broad Raman signal supports the presence of oxygen vacancies and nickelrelated defects. 37A similar behavior of the phonon-related Raman bands in nanosized material was observed in the study by Mironova et al. 38 The shi of all Raman bands to higher wavenumbers indicates increased compressive strain in the lms. 39Notably, the 10 minute sample shows signicant peak shis due to MW heating, with all phonon modes shiing to higher wavenumbers which is associated with the size-induced phonon connement, defects, and surface relaxation. 36The shi in the 2LO peak from 1068 to 1097.6 cm −1 suggests the presence of crystal defects, such as oxygen and nickel vacancies, which can lead to a redshi of the Raman peak.Additionally, the modes broaden with increased MW irradiation due to higher nickel or interstitial oxygen vacancy concentrations, resulting in smaller particle sizes.Raman studies veries that the prepared nanostructure lms have a cubic NiO structure without any impurity or secondary phases, consistent with XRD results.In conclusion, these results imply that MW irradiation induces changes in the defect in the lattice of the material.This highlights the signicant impact of MW irradiation on the material's crystallinity and defects.

XPS analysis of MW irradiated NiO nanostructure lms
The chemical analysis of the sample was further carried out to determine the chemical state of the prepared NiO nanostructured NiO lms.The XPS studies were conducted to analyze the chemical states and to identify the chemical compositions present in the prepared pristine and MW irradiated NiO nanostructure lms.The Fig. 8    No foreign elements were detected in the wide spectrum, indicating the grown lm has the desired NiO composition, consistent with XRD and Raman analyses.To account for the charging effect in the lm on XPS, the binding energies were calibrated using the C 1s peak at 284.8 eV.n the 5 min MW irradiated sample, the Ni 2p 3/2 binding energy peaks are observed at 854.4, 855.2, 860.6, and 864.9 eV.The Ni 2p 1/2 binding energy peaks are observed at 870.8, 872.6, and 878.65 eV.Aer MW irradiation, the core spectra show a shi toward higher binding energies, which is attributed to the formation of defects in the NiO lattice.These defects alter the ionic charge distribution, leading to cation vacancies and changes in the chemical composition, particularly oxygen concentration. 41The peak broadening and shis are also linked to the charging effect, which is induced by microwave irradiation.
The detailed analysis of the decomposed XPS spectrum indicates that the peak intensity for the core levels of Ni 3+ (2p 3/2 and 2p 1/2 ) is higher than that for Ni 2+ (2p 3/2 and 2p 1/2 ) in both pristine and 5 minute irradiated samples.The intensity ratio of Ni 3+ to Ni 2+ peaks (Ni 3+ /Ni 2+ ) increases from 1.924 in the pristine sample to 2.029 in the irradiated sample, indicating a higher concentration of the Ni 3+ state present in the lm.The decrease in the intensity of the core levels for Ni 2+ and Ni 3+ is more noticeable in the 5 minute sample, suggesting the could be due to ionization effects caused by microwave irradiation, which leads to the ejection of electrons from the Ni 2+ and Ni 3+ states.Additionally, defects induced by irradiation can alter the local environment around the nickel ions, reducing the observable intensity of these peaks.Furthermore, the binding energy difference (DE) between the Ni 2p 3/2 and Ni 2p 1/2 peaks is 17.4 eV for both the pristine and 5 minute samples.This conrms that nickel is present in its oxidized forms (Ni 2+ and Ni 3+ ).
Overall, the XPS investigation conrms that nonstoichiometric NiO has a higher proportion of Ni 3+ in both the pristine and 5 minute samples.This analysis supports PL analysis, which also showed an increase in the Ni 3+ state aer irradiation.Therefore, it can be concluded that the lm contains different states of nickel, with a higher concentration of the Ni 3+ state in the nickel oxide lm.
3.3.3.O 1s core spectra.The oxygen core level X-ray photoelectron spectroscopy (XPS) spectra of the oxygen 1s orbital have been analyzed by deconvolution using a Gaussian prole.The spectra were acquired over a binding energy range   of 535 to 525 eV for both pristine and 5 min MW-irradiated NiO nanostructure thin lms depicted in Fig. 10.
The deconvolution process revealed two predominant photoelectric emission bands of O 1s spectra.The pristine NiO thin lms exhibit two distinct peaks located at 528.84 eV and 530.76 eV in the XPS spectra.The peak at the lower binding energy (528.84eV) is attributed to lattice oxygen (O L ) within the NiO matrix.This specic peak signies the presence of oxygen atoms bonded with Ni 2+ ions, forming the Ni-O octahedral coordination characteristic of the cubic rock salt structure. 42e presence of this peak indicates the stoichiometry of the NiO lattice, 43 consistent with the XRD results.
The peak at the higher binding energy (530.76 eV) is associated with oxygen vacancies (O V ) within the lm. 44,45According to U. Kwon et al., 46 the presence of a peak around 531 eV can be attributed to a deciency of Ni 3+ ions and an excess of oxygen from NiO-OH.Similarly, research by Kotta et al. 47   For the 5 min MW irradiated NiO lm, the O 1s spectra exhibit peaks at 529.12 eV and 531.08 eV, respectively.These peaks indicate a shi towards higher binding energies compared to the pristine lm.This shi, known as a chemical shi, reects the alterations in the chemical states of the lm induced by MW irradiation.The higher binding energy peaks suggest changes in the oxidation state and bonding environment of the oxygen atoms within the NiO lattice, highlighting the signicant effect of MW irradiation on the lm's chemical composition.
The area ratio of the oxygen 1s spectra for each peak was calculated, and it indicates that the percentage area of the lattice oxygen peak (O L ) decreased from 61.7% to 54.1% upon MW irradiation.Conversely, the peak associated with oxygen vacancies increased from 38.3% to 45.9%.This change endorses the fact that MW irradiation enhanced the oxygen vacancy defects in the nanostructures, which is consistent with the PL analysis results.

FESEM and EDS analysis of MW irradiated NiO nanostructure lms
The morphology of the nanostructure lms was analyzed using FESEM, which examined the surface morphology of both pristine and MW-irradiated NiO nanostructure lms, as shown in Fig. 11.All prepared lms exhibited smooth, uniform, crack-free surfaces with nanosized grains and distinct boundaries.MW irradiation resulted in relatively smaller and smoother nanograins compared to the pristine lms, indicating that MW irradiation affects the grain size.The reduction in grain size upon irradiation may be associated with increased lattice strain, which restricts lattice growth.This irradiation induces changes in the lm's microstructure, leading to a more dispersed system.
EDS analysis was used to ascertain the elemental composition of NiO nanostructure lms.In Table 3, EDS results for both pristine and MW-irradiated lms indicate the exclusive occurrence of nickel (Ni) and oxygen (O) elements.With increased MW irradiation, the intensity of the O peak rises, indicating an increase in oxygen content in the lms.Specically, samples irradiated for 2 and 4 minutes show the highest oxygen peak intensity, likely due to a higher concentration of defect centers.This excess oxygen may result from the formation of Ni 3+ , which requires additional oxygen for charge compensation. 49Notably, XRD analysis did not detect any Ni 2 O 3 phase in these lms. 50

. AFM analysis of MW irradiated NiO nanostructure lms
The morphology of MW irradiated NiO thin lms was examined using AFM techniques with tapping mode, and 2D and 3D images are presented in Fig. 12.The scan area of the lm is 1 by 1 mm.The AFM analysis of NiO provides clear evidence of morphological changes in the lm, including alterations in grain sizes, ordered arrangement, and lm roughness aer irradiation.The observable change in grain size is evident from pristine to 2 min of MW irradiation.In the AFM image of the 2 min sample, grain is more fragmented into smaller entities and found to be a more ordered way of arrangement than the pristine lm.The morphology of the 3 min samples exhibits a cluster of grains, while the 4 min irradiation results in a distinct formation of well-dened grains.In the case of 5 minutes, a reduction in grain size is observed along with the formation of clustered grains.The 10 min irradiation sample showed an increase in the grain size, attributed to the attributed to the impact of irradiation.This implies that MW irradiation induces modications in the grain distribution within the NiO lm.
Variations in surface roughness were observed with increased MW irradiation.The surface roughness of the lms is inuenced by their crystallinity and texture; larger crystallite sizes generally result in smoother surfaces due to tighter packing of well-shaped grains.Table 4 shows the surface roughness values of the MW irradiated lms.Values of roughness increased for the 2 minute irradiated sample, likely due to the presence of more dislocations and defects introduced by MW heating.However, with further irradiation, the roughness decreased, resulting in smoother surfaces due to reduced grain size and surface diffusion processes. 51

UV-visible analysis of MW irradiated NiO nanostructure lms
The optical properties of the materials, including absorbance and energy bandgap, were assessed using a UV-visible spectrophotometer at ambient temperature.Fig. 13 shows the absorption spectra of both pristine and MW-irradiated NiO nanostructure lms, with an inset providing a detailed view of the irradiation effects on NiO.High absorbance is noted in the UV range of 300-320 nm, whereas the visible and IR regions exhibit low absorbance.The high absorbance in the UV region is attributed to the bandgap absorption of NiO. 52V. Usha et al. 4 suggest that a UV region broad peak is associated with p-p* inter-band transitions.S. Ghazal 53 suggests that the occurrence of this high band is due to transition of the O 2p band to the Ni 3d state of the CB.The absorbance edge for NiO is around 348 nm, shiing to 356 nm aer 10 minutes of irradiation, indicating a redshi associated with bandgap shrinkage.This redshi is associated with the increased presence of defect centers and heightened scattering losses from grain boundaries, ultimately leading to an enhancement in absorption.Additionally, absorbance increases upon irradiation, with the 10 minute sample showing high absorbance at 318 nm.This enhancement is likely due to the increased optical scattering loss of light from the grain boundaries.The increased absorption can also be seen in Fig. 1, likely caused by the enhancement of defects such as oxygen vacancies and increased Ni 3+ ion concentration. 1 , 2 These defect states act as centers, contributing

RSC Advances Paper
to greater light absorption within the material, resulting in increased absorbance as MW irradiation increases.Similar results have been reported in the literature. 3cording to structural investigations, irradiation enhances the defect densities in the lattice of the NiO nanostructure lms.
To conrm this change, we calculated the energy band gap of the pristine and MW-irradiated NiO nanostructure lms, depicted in Fig. 14.The direct bandgap of the NiO lms was determined utilizing Tauc's relation, given as: where a is the absorption coefficient of the lm, hn is the photon energy and A is a proportionality constant.The linear section of the plots was extrapolated to estimate the lm's E g values and are provided in Table 5.The slight variations in the   This incorporation of localized defects causes a decrease in the energy bandgap. 17Furthermore, the reduction in bandgap is also associated with the compressive stress induced by MW heating in the lattice. 54

Photoluminescence analysis of MW irradiated NiO nanostructure lms
Photoluminescence (PL) spectroscopy provides critical insights into the crystal structure and defect densities within thin lms.This technique is highly sensitive to defects, allowing for the detailed characterization of charge excitation and the electronic structure of the material.In our study, we investigated the PL spectra of pristine and microwave-irradiated NiO nanostructure lms at room temperature, using an excitation wavelength of 300 nm.
The PL spectra exhibited a wide range of emissions extending from ultraviolet (UV) to visible regions, indicating the presence of various defects in the NiO nanostructure lms.UV emissions were associated with band-to-band transitions, while the broad and intense visible emissions were attributed to deep- level intrinsic emissions, including nickel and oxygen interstitials and vacancies.
To precisely determine the luminescent emission centers corresponding to these defects, we performed a deconvolution of the PL spectra using a Gaussian distribution function.The results of this deconvolution are illustrated by the solid black line in Fig. 15.The tted parameters obtained from this analysis are summarized in Table 6, providing detailed information on the specic defect states contributing to the observed PL emissions.
The PL spectra of pristine and MW-irradiated NiO nanostructures exhibit two prominent emission bands: one in the ultraviolet (UV) emission and another due to deep-level intrinsic emissions.The UV emission, corresponding to an energy of 3.09 eV, is accredited to near-band edge (NBE) excitonic recombination.The visible emission band, spanning from 2.93 to 2.05 eV, is associated with intrinsic defect levels caused by diverse structural defects, including oxygen vacancies and interstitial defects in the NiO lms.Strong UV emissions arise from direct excitonic recombination near the band edge.F. Chandoul et al. 55 suggest that the origin of this UV emission may be from oxygen vacancies in the NiO samples.The visible deep-level emissions are attributed to point defects, such as vacancies and interstitials within the nanostructured lms. 56,57ll visible transitions fall within the energy bandgap of the lms, as determined from the Tauc plot, conrming that these defects are trapped within the material's bandgap.The deep-  The broad visible deep-level emission centers in MW irradiated NiO nanostructure lms can be classied into two major categories.The rst category includes emissions ranging from violet to wide blue, which are attributed to nickel-related defects within the lms.The second category encompasses broad yellow to orange emissions, associated with oxygen-related defects. 58The energy emissions from 2.93 eV to 2.63 eV are linked to nickel defects, such as interstitials and vacancies in NiO materials.Mochizuki et al. suggest that the emission bands below 3 eV are attributed to nickel-related vacancy defects and occur due to charge transfer between Ni 2+ and Ni 3+ . 59,60The emission at 2.93 eV corresponds to interstitial defects, attributed to the transition of electrons from nickel interstitial sites to holes in the valence band. 58Violet emissions arise due to the presence of nickel interstitial sites and surface defects, correlating with defect density.Blue I (2.80 eV) and blue II (2.63 eV) emission centers correspond to vacancy defects of nickel ions.The 2.80 eV emission is indicative of singly ionized nickel vacancies, 60,61 while the 2.63 eV emission is attributed to doubly ionized nickel vacancies. 62Both transitions occur from nickel vacancy defect states to the valence band.The broad yellow to orange emissions detected in all nanostructured lms, occurring at 2.18 to 2.06 eV, respectively, are attributed to defectrelated emissions arising from oxygen vacancies. 63The 2.18 eV emission, appearing yellow, is attributed to the transition of electrons from the electron donor level to the acceptor level of oxygen vacancies.On the other hand, the orange emission at 2.06 eV is caused by oxygen interstitials. 64Also, the visible  emissions occurred in the lm due to the presence of defects at the grain boundaries, caused by the presence of nickel vacancies or the oxygen interstitials at grain boundaries. 60he effect of microwave irradiation on the position and intensity of defect peaks is illustrated in Fig. 15.While the peak positions remain stable with increasing MW irradiation, the intensity decreases in irradiated samples.A noticeable decrease in the variation of defect peak intensities was observed in the irradiated samples.The increase in irradiation creates defects such as vacancies, interstitials, and other surface imperfections, disrupting the regular lattice structure.These defects serve as non-radiative centers, leading to a decrease in PL intensity by dissipating energy as heat rather than light.Excessive defects further quench PL by forming non-radiative recombination centers. 65Moreover, the PL intensity indicates the dependency of defect density in the lm, resulting in the weak recombination of photogenerated charge carriers.
There are no substantial deviations in the FWHM of the PL emission bands, but the area of the bands decreases with irradiation.The smallest band area was observed in samples irradiated for 4 and 5 minutes, indicating a higher number of defects and a reduction in luminescent centers.Additionally, the relative intensity between NBE emission and deep-level emission bands remains largely unchanged with irradiation.
Aer irradiation, a signicant decrease in intensity was observed.Samples irradiated for 4 minutes exhibited a 75.76% reduction in UV emission intensity.This reduction is attributed to the microwave irradiation enhancing non-radiative defect levels in the lattice.XRD and Raman spectroscopy studies also support the incorporation of non-radiative defect centers upon irradiation.Ni interstitials were found to increase with irradiation, particularly in the 4 minute samples, while doubly ionized Ni vacancies decreased.The area ratio of nickel defects indicated an enhancement in nickel interstitials, with the highest levels observed in the 4 minute irradiated samples.Changes in the yellow-orange emission were also noted, with a decrease in oxygen interstitials and an increase in oxygen vacancies upon irradiation, consistent with XPS results.The calculated area ratios showed that the 5 minute lm had the highest area ratio of 32% for oxygen vacancies, indicating a signicant increase in oxygen vacancy defects.The area ratio quantity of the Ni and O defects were tabulated in Table 7. MW irradiation results in the decrease of PL intensity for NiO nanostructures, indicating that irradiation can effectively inuence luminescence behavior.The quenching of PL emission with increasing irradiation dosage is due to the enhanced incorporation of non-radiative defect centers.
3.8.Third-order nonlinear optical processes of MW irradiated NiO nanostructure lms 3.8.1.Open aperture Z scan technique.The nonlinear optical (NLO) properties of the pristine and microwave irradiated NiO nanostructure lms were determined using a continuous wave (CW) laser with a wavelength of 632.8 nm.The Z-scan experimental method was employed to extract the nonlinear absorption (NLA) and nonlinear refraction (NLR) parameters using the open-aperture (OA) and closed-aperture (CA) approaches.The motivation behind this NLO experiment is twofold: to understand the effect of MW irradiation on NiO nanostructure lms in both pulsed femtosecond laser and continuous wave laser regimes and to identify a suitable NLO material essential for current technological advancements.This section aims to elucidate the impact of continuous wave laserinduced nonlinearity in MW irradiated NiO lms.
The theoretical understanding of NLO phenomena involves the interaction of high-intensity light, such as a laser, with an NLO material.The intensity of the light depends nonlinearly on the electric eld.Consequently, the linear refraction and absorption become nonlinear and rely on the light's intensity as it passes through the material.This relationship is given by: where a 0 and, n 0 , are the linear absorption coefficient and refractive index, respectively.b eff , and n 2 are the nonlinear absorption coefficient and nonlinear refractive index, respectively.I is the intensity irradiance on the materials.The NLA coefficient b eff of the material was determined using the OA Z-scan method, with an input intensity of 20 mW.The obtained results for the open-aperture Z-scan of the lm, which correspond to the far-eld normalized transmittance T(Z) as a function of the sample distance position from the beam focus, are shown in Fig. 17, represent the OA experimental data traces obtained with the Z-scan system.To obtain this graph, we used the general normalized transmittance equation for multiphoton absorption (MPA) in the OA Z-scan t equation.
where q 0 (Z) is the free factor, L eff the effective thickness of the material, m, is the number of photon absorptions.The open-aperture graph exhibits a dip at the focus, indicating positive nonlinear absorption characteristic of reverse saturable absorption (RSA). 23Microwave irradiation enhances  Paper RSC Advances the RSA nature, with the valley signicantly deepening MW irradiation compared to the pristine sample, indicating an increased nonlinear absorption mechanism.The occurrence of RSA can be attributed to several mechanisms, including two-photon absorption (TPA), multiphoton absorption (MPA), free-carrier absorption (FCA), excited state absorption (ESA), transient absorption, or a combination of these nonlinear absorption processes. 66These behaviors are closely related to the bandgap, defect densities, and free carriers present in the lms.The continuous laser irradiation energy of 1.96 eV on the nanostructured lms excites electrons from the valence band to the conduction band through defect levels in the lms, leading to thermal induced excited state absorption.The enhanced and induced defect levels in the lms observed due to MW irradiation are associated with increased nonlinear absorption.In these nanostructures, electrons excited from the ground state to defect levels through photon absorption emit non-radiatively.These trapped electrons can further absorb additional photons, causing excitation to the conduction band, facilitated by ESA.Additionally, the laser irradiation energy of 1.96 eV is insufficient to excite electrons from the valence band to the conduction band through single-photon absorption.The bandgap of the material is ∼3.6 eV, and since the laser photon energy is greater than half but less than the bandgap energy, TPA is possible.The laser energy of 1.96 eV fullls the TPA condition.FCA also plays a crucial role in the nonlinear absorption mechanisms.Microwave irradiation enhances the generation of free carriers within the material, thereby promoting FCA.Despite being a relatively weak nonlinear absorption process, FCA contributes signicantly alongside TPA. 67Although TPA and the FCA-induced TPA are allowed, the high observed absorptive nonlinearity indicates that the RSA is primarily due to thermally induced ESA.Therefore, the RSA process primarily stems from ESA and FCA induced by TPA.
The nonlinear absorption results from the combined contributions of multiple processes, denoted by b eff .The presence and enhancement of defect levels in the lms observed with MW irradiation, identied through structural and  spectroscopic studies, provide additional levels in the bandgap, which enhance the ESA mechanism, leading to RSA in the lm.
Aer MW irradiation of the NiO nanostructured lm, the RSA behavior was found to be enhanced.Using the NLA data, we determined the nonlinear absorption coefficient, which is presented in Table 8.The increase in the NLA coefficient aer irradiation underscores the signicant impact of MW irradiation on the material.This enhancement is attributed to the increased defect levels, including structural imperfections, oxygen, and Ni 3+ defects, which promote excited state absorption along with single, two, three, or multiphoton absorption, leading to NLA processes. 68Additionally, FCA further amplies these mechanisms. 69The enhanced NLA aer irradiation suggests that MW treatment effectively improves the material's nonlinearity.
These results indicate that the enhanced MW irradiation in NiO nanostructures leads to a greater NLA coefficient.It implies that the nonlinear absorption depends on the defect states induced by the irradiation, contributing to NLA and causing stronger NLA behavior in the MW-irradiated NiO nanostructured lms.The results suggest that MW irradiation is suitable for enhancing the nonlinear optical absorption of NiO nanostructure lms.
3.8.2.Closed aperture (CA) Z scan technique.The CA Zscan technique was employed to determine the nonlinear refractive indices of pristine and MW-irradiated NiO nanostructure lms.This method, proposed by Sheik-Bahae, helps calculate the sign and magnitude of the nonlinear refractive indices. 70To utilize the CA method, an aperture with a linear transmittance of 0.7 is placed in front of the detector.The sensitivity to nonlinear refraction is entirely due to the aperture and removing it would eliminate this effect.In the CA Z-scan method, normalized transmittance is plotted against the sample position (Z).In the obtained CA normalized transmittance data, the nonlinear absorption (NLA) effect is presented; it must need to be eliminated from the CA data.To eliminate this effect and acquire pure CA data, the CA data was divided by the OA data, which effectively ruled out the NLA effect.The resulting pure CA transmittance data, plotted against the sample position, is presented in Fig. 18.
The samples showed a pre-focal peak and a post-focal valley signature, indicating negative nonlinear refractive behavior, demonstrating a self-defocusing effect indicative of induced thermal lensing effects.The transmittance for the closedaperture (CA) Z-scan is given below: where Z Z R ; Z is the sample position and Z R is the Rayleigh range.
Df is the on-axis nonlinear phase shi at focus.Normalized transmittance data is presented, with symbols representing the experimental data and solid lines depicting the theoretical transmittance t to (5).Using the nonlinear refraction (NLR) plot of the material, we found the sign of the NLR indices to be negative.The nonlinear refractive index was calculated in both SI units and esu units, represented as g (m 2 W −1 ) and n 2 (esu) respectively, and is given as follows: where c, n 0 , L eff , and I 0 are speed light, linear refractive index, effective length, and intensity at focus, respectively.The values obtained of n 2 obtained from tting the NLR data are presented in Table 8.
The negative nonlinear refractive index in the samples is mainly caused by the thermal effects of the continuous wave laser.The CW laser irradiation on materials induces nonlocal thermal heating, which causes spatial variations in the refractive index of the material.These changes in the refractive index in the materials, which leads to materials to behave as a thermal lens due to the thermo-optic effect. 71The defect state levels in the MW irradiated NiO nanostructure lms act as local thermal centers aer absorbing incident photons and serve as centers for trapping excited electrons, enhancing non-radiative recombination.The non-uniform temperature distribution due to the irradiation causes variations in the refractive index of the MW irradiated material, which behaves like a defocusing lens, further leading to the enhancement in the negative nonlinear refractive index.
In the MW-irradiated NiO nanostructure lm, peak-to-valley separation is greater than 1.7 times the Z R , strongly suggests that the nonlinearity predominantly arises from thermal effects. 72The nanomaterials exhibit absorption at the 633 nm wavelength, as seen in the absorption spectra.This absorption, combined with laser wavelength, leads to resonant nonlinearity, which further supports the presence of thermal nonlinearity in the material.Additionally, the CW laser typically enhances thermal nonlinearity through thermal lensing.Together, these Paper RSC Advances factors conrm the dominance of thermal nonlinearity in this material.Also, we employed the Z-scan technique to explore the effect of the glass substrate impacts the observed nonlinearity.Our results showed that the substrate did not exhibit any nonlinear optical effects, allowing us to eliminate the substrate's inuence on our measurements.
In Fig. 17 and 18, we observe that the experimental Rayleigh length for MW-irradiated samples is narrower than the theoretical Rayleigh length.This is likely due to increased photon absorption processes, such as excited-state absorption (ESA), free-carrier absorption (FCA), and two-photon absorption (TPA).In this case, the nonlinearity is resonant, further amplifying these absorption processes.Additionally, the use of a CW laser contributes to thermo-optic nonlinearity, including thermal lensing, which plays a signicant role in the increase in photon absorption.
The third-order susceptibility is a complex parameter that can be calculated using nonlinear optical parameters such as nonlinear absorption coefficient and nonlinear refractive index.
The real part of the third-order susceptibility can be obtained from the nonlinear refractive indices, and it is given by: With the nonlinear absorption coefficient, the imaginary component of the third-order susceptibility can be computed and expressed as follows: Then, the complex parameter third-order susceptibility can represent as, The below conversion relation helps in interchanging different units, The nonlinear optical parameters, including the nonlinear absorption coefficient (b eff ), nonlinear refractive index (n 2 ), and third-order susceptibilities (c 3 ), were calculated and summarized in Table 8.Microwave irradiation led to an increase in these parameters, such as NLA, which was attributed to induced electronic transitions and the creation of intermediate states in the lms.The variations in the nonlinear refractive index observed are associated with local heating effects induced by the laser light.
An increase in the b eff observed with MW irradiation suggests that MW heating enhances the absorption nonlinearity of the NiO nanostructures.This enhancement is associated with the appearance of structural defects such as oxygen vacancies and Ni interstitials, which further lead to the enhancement of ESA, FCA, and TPA in the nanostructure.These enhancements result in the RSA mechanism being more pronounced, thereby increasing the NLA coefficient.
The higher values of the nonlinear refractive index and thirdorder susceptibility (c (3) ) observed with irradiation suggest the nanomaterial's potential for applications like optical limiting and optical storage.The third-order susceptibility (c (3) ) increased from 1.68 to 5.01 × 10 −2 esu, with the highest value observed for the 5 minute irradiated sample.This improvement is attributed to the reduced surface roughness, which minimizes light scattering and enhances the optical properties.Additionally, the increased number of defect center formations in the 5 minute samples enhances the NLO mechanisms.These enhanced defect densities in nanostructures ndings align with previous analyses such as XRD, Raman, XPS, PL, and UV analysis.A comprehensive Table 9 that summarizes the recently reported NLO parameters for various doped and undoped NiO at CW laser.As far as we know, the current data demonstrates the highest nonlinear parameters for NiO lms under continuous wave prepared using physical and chemical deposition methods to date.This indicates that MW irradiation is an efficient method to enhance the NLO properties of NiO, rendering it a highly suitable material for numerous NLO applications.
3.8.3.Third harmonic generation measurement.In this discussion, we delve into the intriguing effects of thirdharmonic generation (THG) signals concerning the energy density of probing laser beams and their impact on microwave irradiated samples.Fig. 19 serves as a visual representation, illustrating the THG signal's behavior concerning the variation in photoinduced probing laser energy density for both pristine and MW-irradiated NiO nanostructure lms.This exploration sheds light on the complex interplay between laser energy, material properties, and THG responses, offering valuable insights into the underlying mechanisms of nonlinear optical phenomena in nanomaterials.
The THG signal varied with different probing laser beam energy densities from 60 to 150 J m −2 .The plot reveals that there is no signicant THG response for energy densities of the probing laser up to 60 J m −2 .This suggests that below a certain threshold energy density, insufficient energy reaches the sample to generate a visible THG signal.However, as the energy density increases from 60 to 150 J m −2 , there is a gradual enhancement in the THG signal.This enhancement is attributed to the interplay between photoexcitation and relaxation processes at localized trapping levels, a phenomenon also observed by other researchers 77 with increasing power density.
For MW-irradiated NiO nanostructure lms, the THG signal exhibits a non-monotonous trend.Initially, from pristine to 3 minute MW irradiation, the THG signal steadily increases.A signicant enhancement is observed for the 4 minute irradiated lms, followed by a decrease in THG for longer irradiation times.This behavior is explained by multiphoton excitation processes and optimized dipole moments, crystalline growth orientation, nanograin morphology, and optical properties. 78he enhanced THG signal for MW-irradiated lms is attributed to increased optical absorption and excitonic effects.Enhanced optical absorption facilitates greater photon participation in the THG process, while excitonic effects contribute to photoexcitation and relaxation mechanisms.Overall, the interplay of these factors leads to the observed variations in the THG signal.The 4 minute MW-irradiated NiO sample shows a higher THG response than other samples, likely due to conditions favoring THG occurrence, increased photoexcitation and relaxation -Fig.19 The illustration of THG signal with respect to the probing laser beam energy density of MW irradiated NiO nanostructure films.
processes, and multiphoton absorption. 79This sample favors multiphoton excitation over third-harmonic processes, with interface trapping levels playing a signicant role.Additionally, the 4 minute sample exhibits a low surface roughness value, contributing to the higher THG response. 80Furthermore, the laser-induced birefringence process substantially improves the THG signals through laser stimulation. 79These factors collectively contribute to the observed THG enhancement, attributed to improved photoexcitation and relaxation induced by the photo-induced pulsed laser.Non-regular increases in THG were observed by Upadhya et al. 81 spray pyrolyzed Mg-doped ZnO lms, where the enhancement in THG was attributed to the dipole moment of the excited states in the material.Albin et al. 82 studied electron beam-irradiated GaZnO lms under nanosecond and femtosecond laser regimes.They found signicant results for both laser treatments, with enhancement in the femtosecond regime attributed to localized defects and laserinduced refractive index changes.In contrast, the enhancement in the nanosecond regime was primarily due to intraenergy gap states contributing to the NLO mechanisms.Essalah et al. 79 reported a THG experiment with Nb 2 O 5 and observed an increase in THG with rising Nb 2 O 5 content.This enhancement was due to variations in atomic dipole moments, driven by mechanisms such as defect states, multiphoton processes, and additional photo polarization effects that alter the dipole moment of the atoms.Chattopadhyay et al. 83 prepared Cedoped ZnO via the coprecipitation method and found that the THG signal was enhanced when the dopant was in the interstitial position, while the THG signal decreased when the dopant substituted the ZnO nanoparticles.These ndings align with the role of defect states in the observed variations in THG across different materials.Over 400 spots on the sample's surface were averaged in the results.Aer a few milliseconds, the photoinduced THG signal disappeared, demonstrating complete reversibility. 84Only when the photoinduced beam and the fundamental beam overlapped in both space and time was the THG signal recognized.3.8.4.Discussion on MW irradiation on third-order nonlinear optical processes on NiO nanostructure lms.In summary, the third harmonic generation in microwaveirradiated NiO nanostructure lms is signicantly enhanced by femtosecond pulsed lasers.These ultrashort pulses interact with the lm on extremely fast timescales, boosting the nonlinear optical response.The strong THG signal in the 4 minute irradiated sample is due to high irradiation-induced defects, which create energy levels within the bandgap.These defects facilitate transitions from localized states to the allowed band, enhancing the material's nonlinear optical properties.Conversely, the enhanced nonlinearity in MW-irradiated NiO under continuous wave laser irradiation is primarily thermal.The CW laser changes the refractive index through thermal excitation, amplifying the nonlinear response.The 5 minute irradiated sample shows strong third-order nonlinearity linked to exhibits lower surface roughness, resulting in a smoother surface as conrmed by AFM analyses.
Results from THG and Z-scan measurements reveal variations in nonlinearity origins due to the distinct inuences of femtosecond pulsed and CW laser excitations.The femtosecond pulsed regime enhances the nonlinear optical response through ultrafast electronic excitations, making MW-irradiated NiO lms suitable for frequency conversion in high-power laser sources.The CW laser regime's nonlinearity, driven by thermal effects, can be applied in optical limiting materials.These studies show that MW irradiation substantially enhances the nonlinear optical parameters of NiO nanostructure lms, compared to recent data in the thermal nonlinear domain.This conrms MW irradiation as a direct strategy to control and manipulate the nonlinear optical properties of NiO lms.The interplay of ultrafast electronic contributions from the pulsed regime and thermal effects from the CW regime suggests that MW-irradiated NiO nanostructures are promising materials for current nonlinear optical applications.

Conclusion
The pristine and MW-irradiated NiO nanostructure lms with a face-centered cubic structure were synthesized using the chemical spray pyrolysis method.The XRD analysis reveals that crystallite size decreased upon MW irradiation, from 8.26 nm to 7.74 nm, due to the MW heating phenomenon.Raman spectra conrmed the characteristic NiO phonon modes for MWirradiated NiO lms, with a red shi towards higher wavenumbers and increasing intensity for the MW-irradiated samples, indicating increased strain and defect density within the nanostructures.The oxidation states of the NiO were quantied using XPS studies.FE-SEM images conrmed that the nanostructures have a smooth surface with nanograin-like structures.AFM studies showed that variations in surface roughness upon irradiation are due to changes in grain size and the ordered distribution of atoms within the lattice.The direct allowed band gap energies decreased from 3.66 eV to 3.59 eV for pristine and MW-irradiated NiO nanostructure lms, respectively, conrming the narrowing of the band gap.PL quenching is observed in nanostructures with increased MW irradiation, associated with an enhanced quantity of nickel and oxygen defects within the lattice.The THG experiment showed an enhancement in the THG signal upon MW irradiation due to photoexcitation and relaxation processes occurring in the lms, caused by increased defect densities in the lattice.All samples exhibit nonlinear reverse saturable absorption, as revealed by the open aperture Z-scan.Additionally, closed aperture measurements indicate the presence of negative nonlinear refraction, attributed to thermal lensing.The third-order nonlinear optical susceptibility showed an enhancement from 5.4 × 10 −3 esu (pristine) to 4.76 × 10 −2 esu (5 min) upon MW irradiation.The magnitude increased by two orders upon irradiation, implying that MW irradiation is a unique and novel method for enhancing the NLO mechanisms in these materials.Based on the experimental ndings, it is evident that MWirradiated NiO nanostructures demonstrate exceptional nonlinear optical characteristics, which highlights their potential for applications in optical detectors, offering superior protection against harmful laser radiation.

Fig. 2
Fig. 2 Schematic representation of process flow for preparation of microwave irradiated NiO nanostructure films.

Fig. 3
Fig. 3 Schematic representation of CW Z scan experimental setup.

Fig. 5
Fig.5The diffraction spectra of pristine and MW irradiated NiO nanostructure films.

Fig. 6
Fig. 6 SSP plot of pristine and MW irradiated NiO nanostructure films.
represents the XPS survey scan of the pristine and 5 min MW irradiated NiO nanostructure lms.3.3.1.XPS survey.The XPS spectrum revealed photoemission intensity peaks corresponding to Ni 3p, Ni 3s, O 1s, Ni 2p, Auger electron peaks (Ni LMM and O KLL), and the C 1s peak.The binding energies (BE) were as follows: ∼1011 eV for Ni 2s, eV for Ni 2p, the ∼(776-640) eV range for Ni LMM peaks, ∼112 eV for Ni 3s, and ∼67 eV for Ni 3p.The order of these peaks reects the electronic levels of the Ni element, with the inner-shell Ni 2s appearing at a very high BE and decreasing BEs observed as we move to the valence shell electrons.

3 . 3 . 2 .
Ni 2p core spectra.The Ni 2p core level spectra of Gaussian deconvoluted pristine and 5 min MW irradiated NiO nanostructure lms are shown in Fig. 9.The spectra consist of two Ni 2p components: Ni 2p 3/2 and Ni 2p 1/2 .The peak splitting observed in the spectra is due to spin-orbit coupling.As a result of this coupling, the intensities of the split peaks are in a 2 : 1 ratio, which is clearly visible in the survey spectra.This intensity ratio is determined by the number of electrons in each state, reecting the degeneracy of the elements.The binding energy of the Ni 2p 3/2 component ranges from 850 to 868 eV, while the Ni 2p 1/2 component ranges from 868 to 883 eV.In the Ni 2p 3/2 region, four peaks are observed at 853.0, 854.8, 860.2, and 864.6 eV, corresponding to Ni 2+ , Ni 3+ , and the shake-up satellite peaks of Ni 2+ and Ni 3+ , respectively, in the pristine NiO nanostructure lm.The major peak at 853.0 eV and its satellite at 860.2 eV indicate Ni-O bonding in the Ni 2+ state.The peaks at 854.8 eV and its satellite at 864.5 eV are associated with the Ni 3+ state, indicating nickel vacancy defects in the lm.For the Ni 2p 1/2 region, binding energies from 868 to 883 eV are observed.Peaks at 870.4, 872.2, and 878.2 eV correspond to Ni 2+ , Ni 3+ , and the satellite peak of Ni 2+ , respectively.The occurrence of Ni 2+ and Ni 3+ states signify stoichiometric and non-stoichiometric NiO.Increased Ni 3d-O 2p hybridization and d-d transition mixing are the causes of the satellite peaks in the spectrum.

Fig. 8
Fig. 8 XPS survey spectra of pristine and 5 min MW irradiated NiO nanostructure films.

Fig. 9
Fig. 9 Ni 2p core level spectra of pristine and 5 min MW irradiated NiO nanostructure film.

Fig. 10 O
Fig. 10 O 1s core level spectra of pristine and 5 min MW irradiated NiO nanostructure film.
and R. S. Kate et al. 48supports this interpretation.M. Adak et al. also suggest that this peak corresponds to Ni 3+ states in Ni 2 O 3 .The presence of oxygen vacancies has been conrmed by XPS analysis and is corroborated by results from PL spectra.

Fig. 12
Fig. 12 The (a) 2-D images and (b) 3-D images of pristine and MW irradiated NiO thin films.
irradiation, which is associated with band lling.Irradiation increases the number of defect densities, leading to additional energy bands near the valence band (VB) and effectively decreasing the energy band gap.MW irradiation results in the formation of localized defect states, increased defect concentration, breakage of nickel and oxygen bonds, structural disorder, and enhanced strain in the lm.

Fig. 13
Fig. 13 Absorption spectra of pristine and MW irradiated NiO nanostructure films.

Fig. 16
Fig. 16 PL emission band diagram for different defect centers for NiO nanostructure films.

Fig. 17
Fig. 17 OA traces of the pristine and MW irradiated NiO nanostructure film.
Fig. 17 OA traces of the pristine and MW irradiated NiO nanostructure film.

Fig. 18
Fig. 18 CA traces of the pristine and MW irradiated NiO nanostructure film. c

Table 1
Calculated structural parameters of pristine and MW irradiated NiO nanostructure films

Table 3
EDS results of MW irradiated NiO nanostructure films

Table 5
Energy gap values of MW irradiated NiO nanostructure film

Table 6
Summary of PL peak fitted parameters of MW irradiated NiO nanostructure films

Table 7
Area ratio of Ni and O defects of MW irradiated NiO nanostructure films